Unterweger was born in 1950 to Theresia Unterweger, a Viennese barmaid and waitress, and an unknown American soldier whom she met in Trieste, Italy. Some sources describe his mother as a prostitute. His mother was jailed for fraud while pregnant but was released and travelled to Graz, where he was born. After his mother was arrested again in 1953, Unterweger was sent to Carinthia to live with his grandfather and his wife.
Unterweger was in and out of prison during his youth for petty crimes, and for assaulting a local prostitute. Between 1966 and 1975 he was convicted sixteen times, mostly for sexual assault; he spent most of those nine years in jail.
In 1974, Unterweger murdered 18-year-old German citizen Margaret Schäfer by strangling her with her own bra, and in 1976 was arrested and sentenced to life in prison. While in prison, Unterweger wrote short stories, poems, plays, and an autobiography, Purgatory or The Trip to Prison—Report of a Guilty Man, that later served as the basis for a documentary.
In 1985, a campaign to pardon and release Unterweger from prison began. Austrian President Rudolf Kirchschläger refused the petition when presented to him, citing the court-mandated minimum of fifteen years in prison. The campaign for a pardon gathered momentum among Viennese intellectuals, political radicals, writers, artists, journalists and politicians, including the author and 2004 Nobel Prize winner Elfriede Jelinek, Günter Grass, Peter Huemer and the editor of the magazine Manuskripte, Alfred Kolleritsch.
Unterweger was released on 23 May 1990, after the required minimum fifteen years of his life term. Upon his release, his autobiography was taught in schools and his stories for children were performed on the radio. Unterweger himself hosted television programs which discussed criminal rehabilitation, and worked as a reporter for the public broadcaster ORF, where he reported on stories concerning the very murders for which he was later found guilty.
Law enforcement later found that Unterweger killed a prostitute in Czechoslovakia and six more in Austria in 1990, the first year after his release. In 1991, Unterweger was hired by an Austrian magazine to write about crime in Los Angeles, California, and the differences between U.S. and European attitudes to prostitution. Unterweger met with local police, even going so far as to participate in a ride-along of the city's red light districts. During Unterweger's time in Los Angeles, three prostitutes — Shannon Exley, Irene Rodriguez, and Sherri Ann Long — were beaten, sexually assaulted with tree branches, and strangled with their own brassieres.
In Austria, Unterweger was suggested as a suspect for the prostitute murders. In the absence of other suspects, the police took a serious look at Unterweger and kept him under surveillance until he went to the U.S. — ostensibly as a reporter — observing nothing to connect him with the murders.
Law enforcement eventually had enough evidence for his arrest, but Unterweger had left by the time they entered his home. After law enforcement chased him through Europe, Canada and the U.S., he was finally arrested by the FBI in Miami, Florida, on 27 February 1992. While a fugitive, he had called the Austrian media to try to convince them of his innocence. Back in Austria, Unterweger was charged with 11 homicides, one of which had occurred in Prague. The jury found him guilty of nine murders by a 6:2 majority (sufficient for a conviction under Austrian law at the time). On 29 June 1994, Unterweger was sentenced to life in prison without possibility of parole.
That night, he committed suicide at Graz-Karlau Prison by hanging himself with a rope made from shoelaces and a cord from the trousers of a track suit.
Prior to his death, Unterweger asserted his intention to seek an appeal, therefore, under Austrian law, his guilty verdict was not legally binding.
In a 2008 performance, actor John Malkovich portrayed Unterweger's life in a performance for one actor, two sopranos, and period orchestra entitled Seduction and Despair, which premiered at Barnum Hall in Santa Monica, California. A fully staged version of the production, entitled The Infernal Comedy premiered in Vienna in July 2009. The show has since been performed throughout Europe, North America and South America.
In 2015 Elisabeth Scharang directed a film called Jack about Unterweger.
Broad Green Pictures is also developing a film Entering Hades starring Michael Fassbender.
The story of the police investigation, pursuit and prosecution of Unterweger is the subject of an episode of The FBI Files titled "Killer Abroad" (Season 2, Episode 14). He is also the subject of an episode of Biography titled "Poet of Death".
Austrian musician Falco's controversial song Jeanny (Part-I) depicts a murder and rapist's thoughts, and its promotional video contains a number of references to crime scenes both real and fictional; while the "news break" in it (which is also heard in the song) refers in an oblique way to Unterweger, who was still in jail at the time of the single's release.